Alzheimer's disease (AD) is a neurodegenerative disorder affecting 4 million older Americans. Our hypothesis is that brain microvessels contribute to the pathogenesis of AD by producing soluble factors that injure or kill neurons. Recent data from our group strongly support this hypothesis and demonstrate that both brain microvessels isolated from AD patients and rat brain endothelial cells injured in culture secrete factors that cause lethal injury specifically to neurons. We have generated monoclonal antibodies that recognize and immunoprecipitate the rat endothelial cell-derived toxin. Preliminary experiments show that at least one of these antibodies also immunoprecipitates the neurotoxic factor secreted by human AD microvessels, suggesting that the endothelial and microvessel neurotoxins are identical or closely related. These novel findings are exciting and implicate the vasculature as a source of neurotoxins in the brain. The feasibility of achieving the goals of this project, to purify and identify these vascular- derived neurotoxic factors, is strongly supported by our initial data and the availability of monoclonal antibodies. In this project, our specific aims are: 1) to purify and characterize the rat endothelial cell-derived toxic factor (EDTF); 2) to clone the cDNA for EDTF and determine the structural basis for neuronal killing; 3) to characterize multi-functional monoclonal antibodies to the human AD microvessel-derived neurotoxic factor(s); and 4) to determine the localization of neurotoxin(s) in normal and AD tissues. Our experiments are the first to identify the vasculature as a source of neurotoxic molecules in AD. Whether the neurotoxic factor is known or unique, as will be assessed in this project, the demonstration that AD brain microvessels produce soluble factors that injure or kill neurons shows that the cerebral microcirculation is a novel, unexplored source of neurotoxic factors in AD, and a heretofore unrecognized target for therapeutic intervention.